Colorado's voters may have approved Amendment 64, but the federal government is unlikely to allow implementation of the marijuana measure and will probably threaten to sue or begin making arrests to stop it. That's the prediction of Sam Kamin, a University of Denver professor and national expert on the Colorado marijuana scene. He details possible scenarios below.

At the top of our conversation, I asked Kamin to interpret analysis of potential federal responses to Amendment 64 by Brian Vicente, an attorney who's among the measure's most prominent proponents. Turns out Vicente is a former law student of Kamin's -- but that doesn't stop the prof from offering a balanced appraisal of his views.

Vicente told us he feels there's nothing the Justice Department can do to stop the portion of Amendment 64 that decriminalizes adult possession of an ounce or less of marijuana, and Kamin agrees.

"I think Brian's analysis of that is spot-on," he says. "I believe the federal government can do nothing [if Colorado] criminalizes or decriminalizes any particular conduct. If voters want to pass something, they can pass it, and the federal government can't come in and make them undo it."

As such, Vicente believes the A64 provisions most vulnerable to federal action would be those pertaining to the authorization of a retail system for the sale of marijuana for recreational, as opposed to medical, purposes. Kamin concurs about this observation as well, and outlines a possible strategy for a court challenge.

"If the feds seek an injunction against the regulation of retail shops, the lawsuit might look a lot like the United States v. Arizona, the suit they brought to stop Arizona from implementing its immigration bill," he says. In that case, "they essentially said, 'The operation of your immigration system will adversely affect national policy, and where there's a conflict between state and federal law, the federal law prevails.'"

We've included the order on the Arizona suit below.

Why wouldn't the same logic apply to decriminalization? "In regard to criminalizing particular conduct, Colorado is free to follow the CSA [Controlled Substances Act] prohibitions or not. The supremacy clause and the preemption clause prevents us from engaging in conduct that creates an impermissible obstacle to the enforcement of federal law -- so they might not like that we don't have parallel laws prohibiting marijuana. But it doesn't prevent them from coming in and enforcing federal law.

"If we were to encourage the selling of marijuana" by blessing a retail system, though, "they might say that is impermissible," he goes on. "Where they're trying to stop it, we're trying to encourage it, and that conflict can't stand."

At this point, Vicente and Kamin part company.

Continue to read more of our interview with DU professor Sam Kamin about Amendment 64.

A portrait of Sam Kamin.

In Vicente's opinion, the fact that the federal government has allowed the creation of a medical marijuana retail setup -- with the exception of dispensaries within 1,000 feet of schools, dozens of which have been ordered to close or move by U.S. Attorney John Walsh -- gives their argument to act now much less credibility. Kamin isn't so sure about that.

"It's true they could have done this two or three years ago -- that we've had hundreds of retail stores open up since 2009, and they haven't done that," he acknowledges. "They haven't sued to enjoin Colorado from doing that, and they haven't arrested people for doing that. But I could see the federal government telling us, 'We said all along that noncommercial medical use is fine, but this is a totally different thing. There are 100,000-some-odd medical-marijuana card holders in Colorado, but there are four million adults. And the difference between 100,000 and four million adults, plus tourists, is something we can't turn a blind eye to. We gave you a chance to regulate yourself, and you obviously can't, so we're going to come in.'"

Likewise, Vicente believes that the approval of a similar marijuana measure in Washington state would provide an additional argument against the Justice Department lowering the boom. But while Kamin concedes that "it might complicate things from a manpower standpoint," Washington's vote "doesn't really change anything from a legal perspective."

If the federal government wants to act, what are its options?

"There are a couple of things they can do," Kamin notes. "They could simply start arresting people -- close down for-profit centers, and make life more miserable for people running these businesses. People say they don't have the manpower to arrest all of us, but they don't need to arrest everybody. If they shut down one and you're down the block, you'll get the idea."

As for marijuana users, federal agents "could go so far as to start enforcing the Controlled Substances Act for possession -- but they're never going to do that," he feels. "Their interest is in the people making money. The large-scale commercial industry makes them squeamish, and that's what we're supposed to authorize come 2013 or 2014. So unless there's a huge change in policy at the federal level, it's hard to believe we'll go from hundreds of medical marijuana dispensaries to potentially thousands of recreational centers.

"The federal government has been mildly tolerant of medical marijuana, but I can't imagine them saying, 'Go ahead and sell ounces of marijuana to anyone who walks in the door.' I would be surprised if they permit us to fully implement Amendment 64 without a fight."

Not that such a conflict would necessarily resemble an armed invasion of either attorneys or law enforcers.

Continue for more of our interview with DU professor Sam Kamin about Amendment 64. "I don't necessarily think they're going to sue or swoop in with SWAT teams," Kamin maintains. "They might say, 'Look, we'll pretend you didn't pass Amendment 64. We'll let you keep your medical, because we believe there are people sincerely buying and selling marijuana as medical. But if we see you authorizing retail stores [for recreational use], we're going to come in and shut everything down."

Eric Holder.

Kamin points out that this approach is similar to the one U.S. Attorney General Eric Holder took in 2010 when California pushed Proposition 19, a marijuana proposal that failed at the polls. "Holder said, 'If you pass legalization, we'll consider that war -- and California didn't. Now, that was an easier call to make in California, which is a safe state for Democrats, and Obama's name wasn't on the ballot that election. It was harder to do that in a swing state in an election year" -- the scenario in Colorado. "So we didn't see Justice saber-rattle, and we passed what California didn't."

Another possibility floated by Vicente that would allow Amendment 64 to move forward unencumbered: Now that President Barack Obama is guaranteed a second term, and no longer needs to worry about running for reelection, he might tell Holder to back off or even encourage the rescheduling of marijuana -- currently a Schedule I narcotic that is considered to have no medical use. Kamin, though, is extremely dubious about such a development.

As he puts it, "Every liberal I know has this fantasy: Come January 21 [when Obama begins term two], we're going to see the real, pot-smoking, community-activist president we felt we were getting four years ago. But I think he's an inherently conservative guy -- an incremental changer. I'm pretty skeptical about that."

Still, he does offer at least one ray of light for Amendment 64 supporters.

Barack Obama.

"The other parallel I've been talking about is between legalization and decriminalization of marijuana and marriage equality. Both have been on the ballot, both have been growing year by year, state by state. It's been a real generational change, because young people prefer these policies to their seniors. In both, we feel like we're at or near a tipping point, and President Obama came out and surprised a lot of people by endorsing marriage equality. So there is some hope for people in favor of legalization or decriminalization of marijuana that he has one more like that in him. I still don't see him using his capital going into the fiscal-cliff negotiations on this. But it might mean more of a willingness to let the state experiment."

Even so, some problems are built into Amendment 64's timetable no matter what the federal government does. For one thing, possessing an ounce of marijuana will be decriminalized as soon as Governor John Hickenlooper signs the measure, as he's expected to do within thirty days despite his previous opposition to it. (Hickenlooper is reportedly scheduled to speak to Attorney General Holder in a conference call this afternoon on the subject of A64.) Nonetheless, selling marijuana for recreational purposes remains illegal, and that won't change until regulation that's not expected to be finalized until late 2013 or early 2014.

The resulting "one-year cooling-off period" is "like an awkward adolescence," Kamin says. "It's not the end-state but sort of a stopgap while we figure out what the feds are going to do about the retail shops. It does create this black or gray market where people can grow it themselves -- but we don't think most of it is going to come from that. So I think that's a fair criticism. But the alternative was a gold rush to open shops on January 1, 2013. That would really have been chaotic. We wouldn't have known what was going to happen and what people's legal rights were. So I fully understand why [the proponents] took the middle step."

Yet this delay gives the federal government an opportunity to act. Kamin's prediction on that score?

"I think what will happen is, we'll get a statement from Washington. My guess is they'll say, 'You're not permitted to sell recreational marijuana in Colorado and Washington. If you seek full implementation of Amendment 64, we will either sue or arrest.'

"I have no special insights," Kamin emphasizes. "I have no mole in the Justice Department. It's just my sense based on what's gone on in Washington [D.C.] in the last two years in regard to marijuana. It just feels like such an expansion -- to go from serving 100,000 people to serving four or five million -- isn't something they'll stand by and let happen."

Look below to see the 60 Minutes report plus two documents -- a memo by Deputy Attorney General James Cole in regard to the federal approach to medical marijuana dispensaries and the order in the United States v. Arizona.